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Mewaldt, R. A.

Publications and source records attributed to Mewaldt, R. A..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

A reexamination of the cosmic-ray helium spectrum and the He-3/He-4 ratio at high energies

Cosmic ray He spectral data collected by satellites in earth orbit were used to examine a recent measurement of He-3 of about 6 GeV/nucleon, an overabundance compared to predictions made with a leaky box approach. The spectral rigidity index at the time of the measurement was no more than 2.55, indicating a near-earth He-3/He-4 ratio of 0.17, although an index in the range 10-20 GV and a ratio of close to 0.24 is the usual value. New magnetic spectrometer data, however, show that a single spectral rigidity value for the near-earth He-3/He-4 ratio cannot be correct. The Fokker-Planck equation is used to demonstrate that solar modulation of the He flux yields an energy/abundance distribution close to observational data.

Webber, W. R.↗
NASA NTRS1986

He-3 in galactic cosmic rays

Cosmic-ray He-3/He-4 observations, including a new measurement around 65 MeV per nucleon from ISEE-3, are compared with interstellar propagation and solar modulation calculations in an effort to understand the origin of cosmic-ray He nuclei. A survey of spacecraft and balloon observations of the He-3/He-4 ratio shows improved consistency among measurements in the 50-300 MeV per nucleon energy range when a previously neglected contribution from atmospheric secondary He-3 is taken into account. These low-energy observations imply a mean escape length of 6-8 g/sq cm in the standard 'leaky box' model for cosmic-ray propagation in the Galaxy, a value consistent with that derived from studies of heavier nuclei. No evidence is found for an excess of low-energy He-3 such as that reported at high energies.

Mewaldt, R. A.↗
NASA NTRS1986

Solar cycle variations of anomalous He-4 as deduced by studies of cosmic ray He-3

Voyager 1 and 2 observations have been used to derive the energy spectrum of 'anomalous' cosmic-ray (ACR) He using a technique based on the separation of the isotopes of He. Previously reported observations show that the ACR oxygen spectrum underwent a change in shape following the solar magnetic field reversal in 1980; in this paper evidence for a corresponding change in the ACR He spectrum is presented. It is found that the energy spectra of ACR He and oxygen and Galactic-cosmic-ray He-4 and He3 can be reproduced both before and after the field reversal using a simple solar modulation model in which the effects of curvature and gradient drifts are arbitrarily neglected.

Cummings, A. C.↗
NASA NTRS1985

Research in particles and fields

Research activities in Cosmic Rays, Gamma Rays, and Astrophysical Plasmas supported under NASA Grant NGR 05-002-160 are discussed. The report is divided into sections which describe the activities, followed by a bibliography. This group's research program is directed toward the investigation of the astrophysical aspects of cosmic rays and gamma rays and of the radiation and electromagnetic field environment of the Earth and other planets. These investigations are carried out by means of energetic particle and photon detector systems flown on spacecraft and balloons.

Stone, E. C.↗
NASA NTRS1985

Cosmic ray He-3 measurements

The rare isotopes H-2 and He-3 in cosmic rays are believed to be of secondary origin produced by nuclear interactions of primary H-1 and He-4 with the interstellar medium. There has recently been renewed interest in these isotopes as a result of indications from high energy antiproton, positron, and He-3 observations that the origin of some primary H and He nuclei may differ from that of heavier cosmic rays. A new observation of low energy He-3 is examined. Previously reported He-3 and He-4 measurements at both low and high energies are examined and compared with calculations of the expected He-3 and He-4 ratio at 1 AU. No evidence for an excess of low energy He-3 such as that reported at high energies was found.

Mewaldt, R. A.↗
NASA NTRS1985

Measurements of Fe and Ar fragmentation cross sections

Measurements are reported of the yields of individual isotopes of Cr to Co(Z = 24 to 27) resulting from the fragmentation of Fe-56, and the isotopes of Mg to K(Z = 12 to 19) resulting from the fragmentation of Ar-40. Recent advances in the resolution and collecting power of cosmic ray instrumentation, have led to dramatic improvements in the precision of cosmic ray composition measurements, both elemental and isotopic. The interpretation of these measurements is presently limited by uncertainties in the fragmentation cross-sections needed to correct for nuclear interactions with the interstellar gas. Cosmic ray propagation codes now rely mainly on semi-empirical cross-section formulae developed by Silberberg and Tsao (S&T), which have a typical uncertainty of approximately 25%. Relative isotope yields from the fragmentation of approximately 380 MeV/nucleon Fe-56 e and approximately 210 MeV/nucleon Ar-40 r in CH2 targets, observed during the calibration of two cosmic ray spectrometers at the Lawrence Berkeley Laboratory Bevalac are reported. These are compared with calculated yeilds based on the S&T cross-section formulae.

Lau, K. H.↗
NASA NTRS1985

Solar cycle variations of the anomalous cosmic ray component

The intensity of the anomalous cosmic ray component, consisting of He, N, O, and Ne, has long been known to be especially sensitive to the effects of solar modulation. Following its discovery in 1972, this component dominated the quiet time flux of cosmic ray nuclei below approx. 30 MeV/nucleon during the 1972 to 1978 solar minimum, but then became essentially unobservable at 1 AU with the approach of solar maximum conditions. One recent theoretical model predicts substantial differences in the intensity of the anomalous fluxes from one solar minimum period to the next because of the reversal of the solar magnetic field. Using data from the Caltech experiments on IMP-8 and ICE (ISEE-3), the intensity of anomalous O and He at 1 AU during the years 1972 to 1985 is reported in particular. Whether the anomalous fluxes will return to their 1972-1978 levels, as predicted by spherically symmetric modulation models, or whether they will fail to return to 1 AU, as suggested by modulation models in which gradient and curvature drifts dominate are to be determined. The preliminary analysis of data from 1984 shows that the intensity of 8 to 27 MeV/nucleon O is still more than an order of magnitude below its 1972 to 1978 levels, while the intensity of 25 to 43 MeV/nucleon He is a factor of Approx. 8 below its maximum level in 1977.

Mewaldt, R. A.↗
NASA NTRS1985

Research in particles and fields

Research activities in Cosmic Rays Gamma Rays, and Astrophysical Plasmas are covered The investigation of the astrophysical aspects of cosmic rays and gamma rays and of the radiation and electromagnetic field environment of the Earth and other planets are studied. These investigations are carried ut by means of energetic particle and photon detector systems flown on spacecraft and balloons. The emphasis is on precision measurements with high resolution in charge mass and energy. An extensive bibliography is given.

Stone, E. C.↗
NASA NTRS1985

Cosmic ray He3 measurements

Cosmic ray 3He/4H3 observations, including a new measurement at approximately 65 MeV/nucleon from ISEE-3, are compared with interstellar propagation and solar modulation models in an effort to understand the origin of cosmic ray He nuclei.

Mewaldt, R. A.↗
NASA NTRS1985

Measurements of Fe and Ar fragmentation cross sections

Measurements are reported of the yields of individual isotopes of Cr to Co (Z = 24 to 27) resulting from the fragmentation of 56Fe, and the isotopes of Mg to K (Z = 12 to 19) resulting from the fragmentation of 40Ar.

Lau, K. H.↗
NASA NTRS1985

Solar cycle variations of the anomalous cosmic ray component

The intensity of the anomalous cosmic ray component, consisting of He, N, O, and Ne, has long been known to be especially sensitive to the effects of solar modulation. Following its discovery in 1972, this component dominated the quiet-time flux of cosmic ray nuclei below approximately 30 MeV/nucleon during the 1972 to 1978 solar minimum, but then became essentially unobservable at 1 AU with the approach of solar maximum conditions. One recent theoretical model predicts substantial differences in the intensity of the anomalous fluxes from one solar minimum period to the next because of the reversal of the solar magnetic field. Using data from the Caltech experiments on IMP-8 and ICE (ISEE-3), the intensity of anomalous O and He at 1 AU during the years 1972 to 1985 is reported. We hope to determine whether the anomalous fluxes will return to their 1972 to 1978 levels, as predicted by spherically symmetric modulation models, or whether they will fail to return to 1 AU, as suggested by modulation models in which gradient and curvature drifts dominate.

Mewaldt, R. A.↗
NASA NTRS1985

Measurements of cosmic ray isotopes from a space platform

Possibilities for measuring cosmic-ray isotopes with Z = 3-30 from an earth-orbiting space platform are considered. Included are a summary of scientific objectives, a survey of current instrumentation, an examination of the effect of the geomagnetic field, and estimates of the yield of isotopes that could be realized. It is found that space-platform experiments could provide a factor-of-100 improvement in yield over currently approved future experiments. To address the objectives of cosmic-ray isotope studies most effectively, measurements should be made by several complementary instruments spanning a range of energies from less than 0.1 to about 100 GeV/nucleon.

Mewaldt, R. A.↗
NASA NTRS1984

The isotopic composition of the anomalous low-energy cosmic rays

Measurements of the isotopes of low energy cosmic ray He, C, N, O and Ne nuclei are analyzed, in concert with previous data, to examine relationships between galactic cosmic rays (GCR) and anomalous cosmic rays (ACR), which could arise from either extragalactic or interstellar sources. A ground-based heavy isotope spectrometer telescope was used for the survey, along with IMP-7 and IMP-8 data and ISEE-3 data. The data were recorded from 1973-78. Spectral and abundance data are provided for each element. Fragmentation was absent in the ACR data, unlike the GCR data. Differences were also observed in the Ne-22/Ne-20 ratios of ACR and GCR. It is suggested, therefore, that the low energy ACR isotopes have an interstellar origin and are useful for studies of the local interstellar medium.

Mewaldt, R. A.↗
NASA NTRS1984

A high-resolution study of the isotopes of solar flare nuclei

Individual isotopes of the elements He, C, N, O, Ne, and Mg with energies from 5 to 50 MeV per nucleon have been resolved in energetic flare particles during the September 23, 1978 solar flare event. In addition, an earlier determination of Ne-22/Ne-20 in this flare was improved on by extending the energy interval for isotope analysis. A significant difference between the isotopic composition of solar flare and solar wind neon is found, which is compared to similar evidence from studies of solar energetic particles implanted in lunar and meteoritic samples. Although limited by statistics, the measurements of He, C, N, O, and Mg isotopes are consistent with typical isotopic abundances found in other samples of solar system material. The ensemble of these results is used to test for the possibility of mass-dependent fractionation during solar flare acceleration and propagation.

Mewaldt, R. A.↗
NASA NTRS1984

Research in particles and fields

The astrophysical aspects of cosmic rays and gamma rays and of the electromagnetic field environment of the Earth and other planets are investigated. These investigations are carried out by means of energetic particle and photon detector systems flown on spacecraft and balloons.

Stone, E. C.↗
NASA NTRS1984

Reduction and analysis of data from experiment CAI on the IMP-8 mission

The Caltech Electron/Isotope Spectrometer (EIS) on the Interplanetary Monitoring Platform 8 (IMP-8) has provided precise measurements of the energy spectra and time variations of low energy electrons (0.16 to 6 MeV), the isotopes of hydrogen and helium (approximately 2 to 40 MeV/nucleon), and the elements from lithium through oxygen (approximately 5 to 50 MeV/nucleon) in energetic particle fluxes of solar, galactic, interplanetary, and magnetospheric origin since 1973. The accomplishments that have resulted from EIS measurements during the period March 24, 1980 to December 31, 1984 are summarized.

Stone, E. C.↗
NASA NTRS1983

A search for H-2, H-3, and He-3 in large solar flares

The results of a new study of solar flare H and He isotopes imply that earlier observations have significantly overestimated the abundances of H-2, H-3, and He-3 in large solar flares. No evidence is found that solar flare nuclei have suffered any significant amount of fragmentation before escaping from the sun.

Mewaldt, R. A.↗